The present invention relates to a hydraulic drive system. More particularly this invention concerns a hydrostatic transmission for interconnection of a prime mover and a hydraulically operable load.
A drive is known which interconnects a prime mover and a hydraulically operable load and which comprises a variable displacement pump permanently connected to the prime mover and having an outlet side and an inlet side connected via conduits to a hydraulic motor constituting part of the load. With such a system it is therefore possible to operate the prime mover at a substantially continuous rate, while varying the operational speed of the load by varying the displacement of the pump. When the pump is of the reversible type it is also possible, therefore, to reverse the operation direction of the load while still operating the prime mover at the same constant rate. Such an arrangement is often used to drive the wheels of a piece of equipment such as s forklift which must be able to move forward and backward at a variable rate of speed.
It has been suggested to connect in parallel to the variable-displacement pump a constant-displacement pump of the same maximum displacement. Thus it is possible to completely stop the load simply be operating the variable-displacement pump in reverse. Nevertheless such a system has proven in practice to be relatively disadvantageous and starting-up of both the pumps often presents an undue load on the prime mover.